Abstract
The development of quantitative magnetic resonance imaging (MRI) techniques is aimed at increasing the objectivity of image analysis from the absolute measurements and values (1). Quantitative MRI can be fast, standardized, robust, and reproducible (2,3). T2 mapping is a representative technique of quantitative MRI and could be useful when multiple follow-up scans are necessary for comparison, such as during active surveillance of prostate cancer (4). A T2 mapping sequence should be applicable and reliable in a multicenter and multivendor setting and when multiple patient visits are required (4).
Quantitative T2 mapping involves measurement of the tissue T2 relaxation time, which complements the anatomical and spatial information obtained through T2-weighted (T2W) imaging. Both the signal decay curve and T2-relaxation times of T2W spin-echo sequences can be used and presented using a parametric color map (5). It can provide functional information based on the water compartments and proportions of stromal or glandular tissues in normal and malignant prostatic tissues, as well as anatomical and spatial information (6).
A difference in the T2 relaxation times of malignant and non-malignant prostate tissues has been reported (1,3,4,7–11). The T2 relaxation time of the peripheral zone is significantly lower in prostate cancer affected glands than in a normal prostate gland (1,4,7–10). Previous studies showed no significant difference between the T2 relaxation time of the transition zone in malignant and non-malignant prostate tissues (1,4,10); however, a significant difference in this T2 relaxation time has been reported recently (3,11). T2 mapping thus appears useful in identifying prostate malignancy in both peripheral and transitional zones (3,11). In terms of prostate cancer grading, the quantitative T2 values correlate with the aggressiveness of the disease (Gleason score) (3,10).
It has been indicated that the apparent diffusion coefficient (ADC) values and quantitative T2 values are strongly related to cell density. ADC values have a significant inverse correlation with Gleason scores (10). It is likely that quantitative T2 values are inversely correlated with Gleason scores and can provide evidence of tumor aggressiveness. Currently, T2 and ADC values are regarded as biomarkers reflecting the amount of water content, cell density, and tissue composition (12). Both ADC and T2 values may be sufficient for the detection and grading of prostate cancer (10). A major hurdle for the widespread clinical application of mapping techniques is the additional acquisition time (3). Quantitative T2 mapping has not been standardized in clinical routine practice due to long scan times from multiple acquisitions (2). Therefore, the development of an accurate and robust method with acceptable scan times is challenging.
The separate acquisition of quantitative and qualitative images is time consuming, and simultaneous generation of quantitative maps and morphologic images through synthetic MRI may be advantageous (13). Using multi-echo and multi-delay acquisition, synthetic MRI allows for simultaneous quantification of physical properties (14). It is an emerging technique that synthesizes MR images at arbitrary contrast after the actual MR acquisition. In other words, it can provide absolute quantification of T1, T2, and proton density maps simultaneously in a single scan. Quantitative longitudinal relaxation time (T1), transverse relaxation time (T2), and proton density values represent the intrinsic magnetic properties of the tissue and are independent of the MRI scanner or scanning parameters at a given field strength (3). This technique has shown excellent correlation with conventional mapping technique with no inferiority of image quality compared with that of conventional contrast-weighted images (3). The feasibility of synthetic MRI has been shown in breast imaging (14), musculoskeletal imaging (13), and neuroimaging (15). Its utility has been demonstrated for the evaluation of tumor activity in primary prostate cancer as well as bone metastases and metastatic prostate cancer. The diagnostic performance of synthetic MRI is comparable to that of conventional MRI for primary prostate cancer evaluation (detection and grading), in both transitional and peripheral zones (3). Both T1 and T2 values are useful parameters in differentiating prostate cancer from other benign lesions that can be easily confused with prostate cancer in clinical practice (3,11).
MR fingerprinting has been recently introduced into the clinical setting and allows rapid and simultaneous measurement of T1 and T2 in a time-efficient manner (11,16,17). The MR fingerprinting protocol consists of data acquisition, pattern matching, and quantitative map visualization of tissue properties. The tissue properties for the fingerprint are then selected from the dictionary and are used to generate quantitative maps, thereby providing quantitative as well as anatomic information (18). It has been used in various clinical applications, including prostate imaging (11,12,19). Preliminary work shows that MR fingerprinting-based relaxometry in combination with ADC mapping could help characterize both peripheral and transitional zone lesions (11,12,19).
In conclusion, the relaxation maps from synthetic MRI are promising for quantitative measurements with a decreased scan time. Their use may be feasible for the standardized assessment of prostate cancer.
